Several types of electrode exist that are suitable for use in an alkaline electrolyte storage cell such as a pocket type electrode, a sintered type electrode, or a paste type electrode, also known as a non-sintered type electrode. The electrodes in most widespread use at present are of the paste type. Compared with the other types of electrode, a paste type electrode contains a greater quantity of active material, so its capacity per unit volume is higher, and its manufacturing cost is lower.
A paste type electrode is made by depositing a paste either on a two-dimensional conductive support such as an expanded metal, a grid, a cloth, or a solid or a perforated sheet, or else in a porous three-dimensional conductive support such as a felt or a metal or a carbon foam. The main components of the paste are the active material, generally in the form of a powder, and a polymer binder, to which there is often added a conductive material. During manufacture of the electrode, a volatile solvent is added to the paste to alter its viscosity so as to make it easier to shape. Once the paste has been deposited on or in the support, the assembly is compressed and dried to obtain an electrode of the desired density and thickness.
In a paste type nickel electrode, the active material is constituted by a nickel-based hydroxide. Nickel hydroxide is a poor conductor and requires the electrode to include a material that enables electricity to percolate well.
Proposals have also been made to add a powdered conductive material to the paste, for example a cobalt compound such as metallic cobalt Co, cobalt hydroxide Co(OH).sub.2, and/or cobalt oxide, CoO. During the first charge, this compound oxides into cobalt oxyhydroxide CoOOH in which the cobalt is raised to oxidation state +3. This oxyhydroxide is stable in the normal operating range of the nickel positive electrode and it is insoluble in the alkaline electrolyte (KOH). It provides electrical percolation for the electrode.
When stored in the completely discharged state, an alkaline storage cell possessing a paste type nickel positive electrode suffers from its voltage decreasing over time. When stored for more than a few months, cell voltage tends towards 0 volts. Under such conditions, the cobalt oxyhydroxide COOOH reduces slowly. The cobalt is initially taken to oxidation state +2.66 in Co.sub.3 O.sub.4, and then it reaches oxidation state +2 in Co(OH).sub.2.
Unfortunately, cobalt hydroxide Co(OH).sub.2 is very soluble in the electrolyte. Consequently, after a storage period of several months, a loss of conductivity is observed due to the percolation network of the paste electrode being partially dissolved. This gives rise to an irreversible loss of capacity that can exceed 10%. This loss occurs whatever the cobalt compound that was initially included in the paste.
An electrode has been described in which the surface of the particles of nickel hydroxide is covered in cobalt hydroxide. However, during cycling, the cobalt hydroxide diffuses into the particles of nickel hydroxide and the capacity of the electrode decreases. In order to solve that problem, document EP-0 696 076 proposes covering the surface of the nickel hydroxide in a mixture of crystals comprising cobalt hydroxide and a hydroxide of at least one metal selected from aluminum, magnesium, indium, and zinc. That makes it possible to reduce the diffusion of cobalt hydroxide while not eliminating it completely. That solution also suffers from the drawback of making the method of manufacturing the active material more complex.